DICHOTOMOUS THEORY 137 
based on comparisons as to branching and anatomical structure. These 
grounds will not suffice to override the inherent probability that the 
leaves of the Lycopods and Psilotaceae are essentially of the same nature 
as those of the Sphenophylls or Equiseta, and not the consequence of an 
entirely distinct evolutionary history. Moreover, on his own admission the 
“ Prohepatic ” type, from which Professor Lignier’s theory starts, is still wholly 
hypothetical. Further, it may be remarked that the embryology of the 
sporophyte gives no assistance to those who would derive it from a 
dorsiventral thallus. On these as well as other grounds the theory, as 
stated by Professor Lignier, cannot be upheld. 
An essentially similar hypothesis has been enunciated by Tansley (ew 
Phytologist, 1907, p. 25, etc.). He contemplates a megaphyllous origin 
of a Fern-like sporophyte from a “hypothetical Archegoniate Alga,” which 
showed dichotomous branching: certain branch-systems became specialised 
for assimilatory functions as erect shoots, and assumed radial symmetry, 
while the axis originated by transition through sympodial development of 
the dichotomy to monopodial branching. On this hypothesis the dorsiventral 
symmetry would be the primitive and the radial the derivative state in 
the original sporophyte. The megaphyllous types would be primitive and 
from these the microphyllous would be derived by widespread reduction. 
Putting aside the collateral speculations of Tansley to which exception 
may be taken, such as the homoplastic origin of the archegonia and of the 
spores, as well as of the whole sporophyte in Bryophytes and Pteridophytes, 
and the wholesale resort to reduction in order to explain the origin 
of the ancient microphyllous phyla, there are two points of fact, or of 
absence of fact, which appear specially to oppose his theory: he assumes 
a radial type of construction to be derivative for the sporophyte and a 
dorsiventral type to be primitive; but in point of fact, in their individual 
development all sporophytes are originally radial, a condition which has 
probably a very close relation to their production in the archegonium : 
that the dorsiventral state is as a general rule derivative in the sporophyte, 
may be concluded from comparison and shown by experiment (see 
Chapter XVI.). Further, there is no known case of dichotomy in the 
sporophyte, where one branch develops as axis and the other as leaf. 
The known facts derived from living Ferns as well as from the fossils 
point clearly to dichotomous branching of the axis itself and of the leaf 
itself, and to transition from a dichotomous to a monopodial branching 
in the establishment of rachis and pinna. But such evidence is wanting 
in the relations of leaf and axis. It was chiefly the absence of such 
evidence that influenced me in rejecting my own suggestion of origin of 
the shoot from a dichotomous branch-system made in 1884 (Pil. Trans. 
vol. ii, 1884, p. 605): it applies equally to the theory as stated by 
Tansley, which appears thus to break down on the test of fact. 
There remains the third view, which, however, is no new one; for 
there have. not been wanting those who have assigned a more prominent 
